A Seismic Investigation of Uturuncu Volcano and the Lazufre Volcanic Complex

The following dissertation is a study of three seismological techniques used to determine the geophysical properties of two large, inflating magma bodies in the upper crust in South America: one under Uturuncu volcano and one beneath Lastarria and Cordon del Azufre volcanoes. First, I use the method of teleseismic receiver functions to image the top and bottom of the magma body beneath Uturuncu volcano. Depths to the top of this body vary between 6 and 12 km below sea level, while depths to the bottom vary between 13 and 22 km below sea level, with the thickness ranging from 6 to 15 km. Then, I compute hypocenters and classify the earthquakes occurring between Lastarria and Cordon del Azufre volcanoes to determine if a magma body between them is priming for eruption or if it may be feeding a shallower magma chamber beneath Lastarria. My classification scheme consisted of five types of seismic events: volcano-tectonic, long period type 1, long period type 2, hybrid, and unknown. The majority of these events are above 10 km below sea level, and most are near Lastarria. I determine that this seismic activity is due to mobile gases and fluids. Finally, I use an attenuation method to study the heterogeneity of the crust between Uturuncu volcano and the magma body beneath it. The assumptions of the method used are undermined by the variability of the earthquake sources and their spectra as well as by extreme geological heterogeneity of this location

Q-values and Attenuation of the Shallow Crust Under Uturuncu Volcano, Bolivia

Uturuncu Volcano, located in the Altiplano-Puna region of the central Andes, near the border of Bolivia and Chile, has been shown to be inflating at a rate of 1-2 cm/yr over an area that is about 70 km wide. The PLUTONS project deployed 28 broadband seismometers around Uturuncu from April 2009 to October 2012. Several thousand shallow (depth \u3c 20 km) local earthquakes were recorded. Attenuation of seismic waves along travel paths for these local crustal earthquakes can be measured by calculating Q-values, which we have performed using the method of single station spectral ratios by Frankel (1982). Large scatter in the Q-values for various distances and travel times appear to be a function of variations in source depth, focal mechanism, and back azimuth. Preliminary Q-values were calculated for azimuths in 30° increments in sectors around each station. Estimates for Q range from about 60 to 700, with many showing a low Q in the direction of the summit from each station. This suggests that the volcanic pile is more highly attenuating than the surrounding crust

Characterizing the Seismic Activity of the Lazufre Volcanic System in Northern Chile/Argentina

The Lazufre Volcanic System (LVS), on the border of Northern Chile and Argentina, is an active complex comprised of two volcanoes, Lastarria to the north and Cordon del Azufre to the south. This system has been showing two unusual inflation signatures based on InSAR. The first, a shallow body between 1 km above sea level and 2 km below sea level is thought to be a shallow magma body underlying Lastarria Volcano. The second source of inflation between Lastarria and Cordon del Azufre is shallowest at approximately sea level, extends as deep as 40 km below sea level, and may merge with the Cerro Galan Magma Body to the east. Current known activity consists of seismicity, extensive and highly active fumarole fields, and degassing. Gas compositions were shown to have changed between 2009 and 2012 from a hydrothermal nature to a magmatic composition. The PLUTONS Project deployed 8 broadband seismic stations throughout the LVS between November 2011 and March 2013. Events within 20 km of the seismic array have been classified as volcano-tectonic (VT), long-period 1 (LP1, with dominant frequency of ~6 Hz and preliminary depths above 1 km below sea level), long period 2 (LP2, with dominant frequency of 3 Hz and preliminary depths between 1 and 10 km below sea level), hybrid, and a new hybrid event type we have termed \u27glissando events\u27. The glissando events start with low frequencies and gradually glide to higher frequencies. The events in all classes do not correlate temporally with large regional or teleseismic earthquakes, and their epicenters are mainly localized near the summit of Lastarria. Using an updated velocity model and allowing for hypocenters to be as shallow as the summit of Lastarria, we re-ran the STA/LTA detector and associator with parameters empirically determined for the identified event types. Preliminary manual review of the resulting database shows that the hypocenters are generally shallower than 10 km below sea level. For each day, in addition to the \u3e5 locatable, local events per day, we found dozens of smaller events. The rates were relatively stable throughout the study period. These preliminary findings suggest moderately high volcanic and/or hydrothermal activity at the LVS. The VT, LP1, LP2, and hybrid type events are similar to those seen at other volcanoes, while the glissando events are not in the literature

Seismicity of the Lazufre Volcanic System, Northern Chile/Argentina

Cordon del Azufre and Lastarria are two volcanoes on the border of Chile and Argentina that have been shown to have an unusual deformation signature. The main center of deformation is between the two volcanoes, with the top of the modelled deformation source at a depth of 7-15 km. A smaller deformation signal is near the summit of Lastarria, with a deformation source at ~1 km below the surface. The most recent eruptions at these volcanoes occurred ~150 ka at Lastarria, and ~0.3 Ma at Cordon del Azufre. Current geologic activity, which is not centered on just the volcanoes, but also in between the two, consists of fumarolic activity, seismic activity, and degassing with a possible change in gas composition between 2009 and 2012 that may be indicative of a change from a hydrothermal to a magmatic nature. The PLUTONS Project deployed 8 seismic stations around the Lazufre complex from November 2011 to March 2013. Preliminary results from analyzing the seismic activity show an average of 1 autodetected event per day within a 20 km radius of Lazufre with the majority of depths less than 10 km below sea level. The average magnitude is ML~1, and does not exceed ML = 2.8. The b-value for these events is ~1.03 ± 0.08 (a typical tectonic value), with a magnitude of completeness threshold Mc = 0.3. The peak event rate is 17 events per day in the beginning of May 2012. These results suggest moderately high volcanic and/or hydrothermal activity at the Lazufre Volcanic complex, with most events occurring beneath the summit of Lastarria